Ventricular defibrillation is a life-saving procedure whose improvement would benefit the hundreds of individuals in whom it is attempted every day. The objective of this grant is to improve ventricular defibrillation by increasing our knowledge about the basic principle of how electric fields interact with fibrillating myocardium. This basic knowledge should allow the reasons for failure of a defibrillation shock to be ascertained, leading to the logical, systematic improvement of defibrillation techniques by overcoming the reasons for failure. A specially-constructed cardiac mapping system will be used to measure ventricular electrical activity during fibrillation until just before the shock, the distribution of potentials throughout the heart generated by the shock, and then the myocardial response a few milliseconds after the shock, simultaneously from 128 electrodes on and in the heart of animals. This information will be used to accomplish five specific aims: 1. to determine the mechanism by which a shock halts fibrillation; 2. to determine why shocks of equal energy in the same animal sometimes succeed and sometimes fail to halt fibrillation; 3. to determine the minimum and maximum values of the electrical field strength of the shock that result in successful defibrillation without causing myocardial damage; 4. to determine the best electrode configurations for external and internal defibrillation defined as the configurations that minimize the shock energy, by creating fields throughout the ventricles that are everywhere within the maximum and minimum values determined as part of Specific Aim 3; 5. to determine the mechanism by which biphasic shocks are more efficacious for defibrillation than monophasic shocks. The accomplishment of these specific aims should improve all types of defibrillation by raising success rates and reducing cardiac injury, and should improve implantable defibrillators by decreasing energy requirements so that they can be made smaller and can deliver the shock sooner after the onset of fibrillation.